Just as a quartz crystal vibrates inside a quartz watch to give us a constant
frequency from which to measure time, so to, do atoms. The first quartz clocks
built in the 1920s taught us that the earth didn't actually rotate at a constant
speed from day to day. Prior to the laboratory quartz clock we were unable to
keep time accurately enough to be able to make such a measurement. So now we
knew that to define one second as being 1/86,400 of a solar day was nonsense.
We needed something that was more consistent than the turning of the earth on
which to base our new standard unit of time, we needed an atomic clock.

Each of the atoms of the periodic table emit electromagnetic radiation at its
own characteristic frequency. Scientists believe that these frequencies are
constant over time and that the frequency of a cesium atom today is still the
same as it would have been millions of years ago. Measuring the atomic vibration
of particles would be our replacement for the quartz crystal, resulting in a
clock many millions of times more accurate.

War time experimentation and development in radar and microwave radio transmissions
had advanced our knowledge to the level required to build the first atomic clock.
Microwaves would be needed to excite the atoms of an element and in1949, early
American experiments were focused on the ammonia molecule. Sadly the results
were not much better than for laboratory quartz clocks and so attention quickly
moved to the cesium clocks being tested in Europe.

In 1955, the world's first atomic clock, based on cesium, was built in England
by the National Physical Laboratory in conjunction with the US Naval Observatory.
They were able to successfully measure the resonant frequency of cesium-133
or more accurately measure the frequency relative to astronomical time. More
cesium testing devices were built in America for cross-referencing the results
and finally, in 1967, the world was ready to change their reference of time
from the rotation of the earth to the resonant frequency of cesium-133. The
second was redefined as being exactly 9,192,631,770 oscillations of the cesium
atom's resonant frequency. These new generation atomic clocks are reported to
keep time to within 30 billionths of a second per year.

How does the cesium atomic clock work?

Cesium-133 atoms are first heated to a gas inside the clock. A high velocity
beam of cesium-133 atoms escape from the super-heated clock and are directed
towards a powerful magnetic field. This magnetic field separates absorption
cesium atoms from their opposite state, emission cesium atoms. The absorption
cesium atoms are then beamed into a microwave oven emitting microwaves at a
frequency very close to that of cesium-133. Some of these atoms then absorb
microwaves and are redirected towards a detector. By amplifying this frequency
and feeding it back to the microwave oven the microwaves can be tuned to the
exact frequency of cesium-133, concentrating the number of energized atoms that
reach the detector. Once the exact frequency of cesium-133 has been achieved
by the microwave oven the frequency can be downloaded and reduced in frequency
and used to keep time to immense accuracy.

The theory of the atomic clock, simply expressed, all atoms of cesium-133 are
exactly the same and vibrate at the same frequency and therefore, collectively,
make an ideal clock, more accurate than we have ever seen before.